(1) In the field of metal oxide semiconductors (MOS memory), RAM (random access memory) is used when data has to be rewritten at high speed, and ROM (read only memory) is used when memory of data is more important than rewriting data. In SRAM and DRAM, part of a device in which data is stored is in conjunction with a Si substrate, and therefore the charges stored as the data leak, so that SRAM and DRAM require some mechanism for compensating for the leakage of the data. In the case of SRAM, the recovery of leaked data is carried out by supplying charges from a DC power source, while in the case of DRAM, leaked data is recovered by an intermittent refresh operation. In contrast, in ROM, in particular, in mask ROM in which data is written by a wafer process, writing is carried out only once, but there is no limitation on the memory of data. Memories which are collectively called "non-volatile memory" are memories in which writing can be carried out by the user and no power source is required for retaining data.
The following various types of non-volatile memories have been developed in accordance with the limitation on the number of times for rewriting, and the rewriting mode: 1. OTPAOM, 2. EPROM, 3. FLASH, 4. EEPROM, 5. NVRAM, and 6. FRAM.
The following Table 1 shows the differences in the performance level between an ideal memory and currently employed memories:
TABLE 1 ______________________________________ Current Memories Ferro- electric Ideal Memory Material Others ______________________________________ Number of Times &gt;10.sup.15 times -10.sup.11 times &lt;10.sup.6 times for Rewriting Rewriting Speed &lt;100 nsec -100 nsec &gt;1 .mu.sec Memory Time &gt;10 years &gt;10 years &gt;10 years Period Capacitance &gt;64M 16K 4M ______________________________________
From the above-mentioned point of view, FRAM using a ferroelectric material is considered closest to the ideal memory, although FRAM is limited with respect to the capacitance, and the number of time for rewriting. Furthermore, it is well expected that the problem with respect to the capacitance of FRAM can be solved in the not long distant future by the simplification of the design rule and the further development of the process technologies therefor. For these reasons, FRAM is currently studied with utmost efforts as the only memory with the expectation that FRAM will make an ideal memory in the future.
FRAM has been reported, for instance, in the following references:
1: F. Gandiner: Ramtron's Technical Report (Uni-RAM).
2: S. S. Eaton et al: Tech. Dig. of ISSCC (1988) P. 130 (Shadow SRAM).
3: R. Moazzami et al: IEEE Electron Device Lett., vol. 11, No. 10 (1990) P. 454 (FNVARAM).
The problem to be solved with respect to FRAM which uses a ferroelectric film is the number of times of rewriting. Although this cannot always be said decisively, it is said if FRAM is improved to the extent that the rewritable number of times thereof exceeds 10.sup.13, FRAM will acquire a firm footing for its use in practice by incorporating it in a current system.
The deterioration of the characteristics of a ferroelectric memory during repeated rewriting operations, that is, the fatigue of FRAM, is caused, for instance, by the fatigue thereof by repeated polarization reversal, the diffusion of the materials for the electrodes into the ferroelectric film, the dispersion of oxygen atoms within the lattice of the ferroelectric thin film, and the generation of space charges by the dispersion of oxygen atoms.
(2) The technologies of recording information by use of light, such as photograph and movies, have been indispensable technologies for our daily life. Studies of high density optical disks have begun around 1972. Recording systems using optical recording media and laser as the light source thereof have attracted attention as being recording systems with an unconventionally large capacity such as 500 M bytes to several G bytes per medium and therefore are currently studied actively all over the countries. Such optical recording media have various features. Representative features thereof are as follows:
1. Capable of recording information with high density and an extremely large capacity. Having a recording density of 10.sup.8 bit/cm.sup.2 or more for recording and reproducing information by a laser beam with the diameter thereof being narrowed down to 1 .mu.m, and having the possibility of increasing the recording density to 1,000 times by shortening the wavelength of the laser beam, and by the multiplication of the recording wavelength.
2. Being resistant to dust even with a particle size of a micron order, since recording and reproduction can be carried out with an optical head sufficiently being out of contact with the recording medium, and accordingly there being no risk of the head crash. Exchanging and hand carrying the recording medium being simple and easy.
Currently employed optical recording media can be roughly classified into (a) a read only type, (b) a write-once type, and (c) a rewritable type. In view of the developed processes of these optical recording media, the rewritable type is considered to be one of optical recording media for the next generation and is being studied as such a memory medium.
As the optical recording media for the next generation, for instance, magneto-optical recording media, phase-changeable recording media, and organic-material-based optical recording media have been reported.
Of these recording media, magneto-optical recording media have been most developed in view of the use in practice. However, in view of the practical use thereof in the future, it is required that the magneto-optical recording media be improved with respect to the quality of signals so that they can handle wide-band analog signals such as signals for use in video disks. To be more specific, it is required that the magneto-optical recording media be improved so as to be able to obtain a sufficiently large Kerr rotation angle (.theta.k) in a short wavelength region.
The development of materials for the magneto-optical recording media appears to reach the ceiling since amorphous magnetic materials are mainly used for the magneto-optical recording media, and no better materials have been discovered for them. Furthermore, with respect to the operational reliability thereof, the amorphous magnetic materials extremely lack resistance to the environmental conditions and need various protective coatings for use in practice. Magneto-optical disks currently commercialized utilize a magnetic film, but have the shortcoming that overwriting cannot be carried out as in magnetic disks.
Under the above-mentioned circumstances, phase-changeable type optical recording media are currently in the process of development. The phase-change type optical recording media whose structure is reversibly changed by the application of a laser beam thereto have in principle the following advantages over conventional magneto-optical recording media:
(i) No external magnetic field is necessary for recording and erasing information.
(ii) Large production signals can be obtained because the recorded state in the recording media can be directly detected in the form of the quantity of light reflected.
In the phase-change type optical recording media, the phase is changed from an amorphous phase to a crystalline phase and vice versa, and chalcogen compounds are mainly used as materials capable of performing such phase changes.
The characteristics required for the material used in the phase-change type optical recording media are as follows:
(a) Having high crystallization speed (not more than 100 nsec).
(b) Maintaining an amorphous state at room temperature in a stable manner for an extended period of time (more than 10 years).
(c) Having a large erasure ratio (not less than 30 dB).
(d) Having a high repeated recording and erasure durability (not less than 10.sup.6 times).
However, a material which meets the above requirements has not yet been developed and a long period of time will be required before a material which satisfies the above requirements is developed.
Furthermore, a repeated recording and erasure durability of at least 10.sup.9 is required for backing up a large scale computer. There is a view that such a durability cannot be achieved by phase-changeable materials.
On the other hand, organic recording media attracted attention as optical memory media when they were first developed. Those organic recording media are of a "read only" type and of a "write-once type" and appear to be already old-fashioned.
Recently it has been reported that photochromic and photochemical whole burning (PHB), using dye materials, will provide a rewritable medium. However, the studies on such materials have just begun, although they appear promising.
Furthermore, the use of a PLZT electro-optical thin film which is a ferroelectric material as a recording layer for an optical recording medium has been proposed in C. E. Land "Proceeding of the Symposium on Thick and thin Films" Am. Ceram. Soc., Indianapolis, 1989. 4, P. 343-359. However a specific device using a PLZT thin film has not yet been reported, perhaps because of some difficulties in the production technique for producing a PLZT thin film and some limitations encountered on the materials employed therein.
(3) An antiferroelectric-to-ferroelectric phase transition is induced in a zirconium oxide-tin oxide-lead lead based ceramics by heat, stress or electric field applied thereto. Of the phase transitions induced by heat, stress and electric field, the distortion induced by electric field produces a larger displacement than the displacements caused by an inverse piezoelectric effect or an electrostrictive effect of a piezoelectric material and characteristically has a digital nature.
Furthermore, the electric-field-induced displacement is isotropic, so that an anisotropic stress concentration can be avoided. Therefore materials capable of performing the antiferroelectric-to-ferro-electric phase transition are expected to be less destructed than piezoelectric materials.
It has been proposed to apply materials capable of performing the antiferroelectric-to-ferroelectric phase transition to a micro-displacement control device and to a shape memory device by utilizing the changes in the volume caused by the distortion and the shape memory characteristics.
In order to realize an intelligent device by highly integrating these devices, a technique for producing a thick film of a material capable of performing antiferroelectric-to-ferroelectric phase transition, and a fine working technique for the film are required. However, nothing has been reported about these techniques.
Ceramics have conventionally been produced by mixing metal oxides which constitute ceramics in the form of a slurry and by sintering the slurry at high temperatures.
A large ceramic film can be produced by this method and the raw materials for use in this method are inexpensive. However, it is extremely difficult to closely control the formulation of the compositions for producing a ceramic film by this method, because the compositions have to be sintered at high temperatures in order to produce a film with high density and the desired high performance. Furthermore, the ceramics produced by this method are generally in the shape of a block or a sheet, and it is difficult to produce a thin ceramic film by this method.
Thin films of ceramics are generally produced by various methods such as vacuum deposition, laser abrasion, sputtering, ion-plating, CVD, MO-CVD, and sol-gel process. Of these methods, MO-CVD, laser abrasion, and sol-gel process are considered suitable for producing multi-component composite oxides because the composition thereof can be controlled relatively precisely.
Laser abrasion can provide crystalline films at a relatively low substrate temperature, but cannot currently provide large ceramic films.
MO-CVD has the advantages over other methods that coating performance is excellent with any shapes, the film formation speed is high, and the control of film thickness is easy, but has some limitation on the availability of materials suitable for use in this method.
In the case of sol-gel process, the film formation can be carried out in the open atmosphere and a large ceramic film can be easily produced, but the coating performance is not so good as that of MO-CVD. A remarkable point about this method is that this process can easily cope with the increase of the constituent elements of ferroelectric materials.
The inventors of the present invention have investigated a method of producing a thick ceramic film by utilizing a sol-gel process, taking into consideration the fact that this process can easily increase the components for a ceramic film to be made.
Sol-gel process is conventionally widely used for producing SiO.sub.2 glass films and plates with various thicknesses. However, with respect to crystalline metal oxide films, the thickness of a crack-free film obtained by one film formation process is at most about 500 .ANG., so that this film formation process has to be repeated many times in order to obtain a thick film of a crystalline metal oxide film. Therefore, it is in fact difficult to produce a thick film of a crystalline metal oxide film by a sol-gel process and a method of producing such a thick film by utilizing a sol-gel process has not yet been reported.